[css-d] child margin affecting parent?

[COM]

Given the css below, why should the header margin value also push the wrapper 
downward? In my markup, the wrapper div is the parent, margin:0, auto;

thank you

J

*{
padding:0;
margin:0;
}
body{
        background-color:rgb(200,200,200);
        font-family:helvetic-neue, Verdana, Arial, sans-serif;
        font-size:100%;
}
#wrapper{
        margin:0 auto;
        width:940px;
        padding:0 20px;
        background-color:rgb(220,230,220);
}
header{
        margin:20px 0 1em 0;
        border:red 1px solid;
}
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